The invention relates to permanent magnets including rare earth elements, iron and boron as primary ingredients, and more particularly to an anisotropic rare earth-iron series permanent magnet having a columnar macrostructure.
Permanent magnets are used in a wide variety of applications ranging from household electrical appliances to peripheral console units of large computers. The demand for permanent magnets that meet high performance standards has grown in proportion to the demand for smaller, higher efficiency electrical appliances.
Typical permanent magnets include alnico magnets, hard ferrite magnets and rare earth element--transition metal magnets. In particular, good magnetic performance is provided by rare earth element --transition metal magnets such as R-Co and R-Fe-B permanent magnets.
Several methods are available for manufacturing R-Fe-B permanent magnets, including:
1. A sintering method based on powder metallurgy techniques;
2. A resin bonding technique involving rapidly quenching ribbon fragments having thicknesses of about 30.mu.. The fragments are prepared using a melt spinning apparatus of the t used for producing amorphous alloys; and
3. A two-step hot pressing technique in which a mechanical alignment treatment is performed on rapidly quenched ribbon fragments prepared using a melt spinning apparatus.
The sintering method is described in Japanese Laid-Open Application No. 46008/1984 and in an article by M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto and Y. Matushita that appeared in Journal of Applied Physics, Vol. 55(6), p. 2083 (Mar. 15, 1984). As described in the article, an alloy ingot is made by melting and casting. The ingot is pulverized to a fine magnetic powder having a particle diameter of about 3.mu.. The magnetic powder is kneaded with a wax that functions as a molding additive and the kneaded magnetic powder is press molded in a magnetic field in order to obtain a molded body. The molded body, called a "green body" is sintered in an argon atmosphere for one hour at a temperature between about 1000.degree. C. and 1100.degree. C. and the sintered body is quenched to room temperature. The quenched green body is heat treated at about 600.degree. C. in order to increase further the intrinsic coercivity of the body.
The sintering method described requires grinding of the alloy ingot to a fine powder. However, the R-Fe-B series alloy wherein R is a rare earth element is extremely reactive in the presence of oxygen and, therefore, the alloy powder is easily oxidized. Accordingly, the oxygen concentration of the sintered body increases to an undesirable level. When the kneaded magnetic powder is molded, wax or additives such as, zinc stearate are required. While efforts to eliminate the wax or additive are made prior to the sintering process, some of the wax or additive inevitably remains in the magnet in the form of carbon, which causes the magnetic performance of the R-Fe-B alloy magnet to deteriorate.
Following the addition of the wax or molding additive and the press molding step, the green or molded body is fragile and difficult to handle. This makes it difficult to place the green body into a sintering furnace without breakage and remains a major disadvantage of the sintering method.
As a result of these disadvantages, expensive equipment is necessary in order to manufacture R-Fe-B series magnets according to the sintering method. Additionally, productivity is low and manufacturing costs are high. Therefore, the potential benefits of using inexpensive raw materials of the type required are not realized.
The resin bonding technique using rapidly quenched ribbon fragments is described in Japanese Laid-Open Patent Application No. 211549/1983 and in an article by R. W. Lee that appeared in Applied Physics Letters, Vol. 46(8), p. 790 (Apr. 15, 1985). Ribbon fragments of R-Fe-B alloy are prepared using a melt spinning apparatus spinning at an optimum substrate velocity. The fragments are ribbon shaped, have a thickness of up to 30.mu. and are aggregations of grains having a diameter of less than about 1000.ANG.. The fragments are fragile and magnetically isotropic, because the grains are distributed isotropically. The fragments are crushed to yield particles of a suitable size to form the magnet. The particles are then kneaded with resin and press molded at a pressure of about 7 ton/cm.sup.2. Reasonably high densities (-85vol %) have achieved at the pressure in the resulting magnet.
The vacuum melt spinning apparatus used to prepare the ribbon fragments is expensive and relatively inefficient. The crystals of the resulting magnet are isotropic resulting in low energy product and a non-square hysteresis loop. Accordingly, the magnet has undesirable temperature coefficients and is impractical.
Alternatively, the rapidly quenched ribbons or ribbon fragments are placed into a graphite or other suitable high temperature resisting die which has been preheated to about 7000.degree. C. in vacuum or inert gas atmosphere. When the temperature of the ribbon or ribbon fragments is raised to 700.degree. C., the ribbons or ribbon fragments are subjected to uniaxial pressure. It is to be understood that the temperature is not strictly limited to 700.degree. C., and it has been determined that temperatures in the range of 725.degree. C..+-.25.degree. C. and pressures of approximately 1.4 ton/cm.sup.2 are suitable for obtaining magnets with sufficient plasticity. Once the ribbons or ribbon fragments have been subjected to uniaxial pressure, the grains of the magnet are slightly aligned in the pressing direction, but are generally isotropic.
A second hot pressing process is performed using a die with a larger cross-section. Generally, a pressing temperature of 700.degree. C. and a pressure of 0.7 ton/cm.sup.2 are used for a period of several seconds. The thickness of the material is reduced by half of the initial thickness and magnetic alignment is introduced parallel to the press direction. Accordingly, the alloy becomes anisotropic. By using this two-step hot pressing technique, high density anisotropic R-Fe-B series magnets are provided.
In the two-step hot pressing technique which is described in Japanese Laid-Open Patent Application No. 100402/1985, it is preferable to have ribbons or ribbon fragments with grain particle diameters that are slightly smaller than the grain diameter at which maximum intrinsic coercivity would be exhibited. If the grain diameter prior to the procedure is slightly smaller than the optimum diameter, the optimum diameter will be realized when the procedure is completed because the grains are enlarged during the hot pressing procedure.
The two-step hot pressing technique requires the use of the same expensive and relatively inefficient vacuum melt spinning apparatus used to prepare the ribbon fragments for the resin bonding technique. Futhermore, two-step hot working of the ribbon fragments is inefficient even though the procedure itself is unique.
Accordingly, it is desirable to provide improved methods of preparation of rare earth-iron series permanent magnets that minimizes the disadvantages encountered in these prior art methods.